WITH query1 AS (
    SELECT LAG("action") OVER (PARTITION BY client_id ORDER BY hitdatetime) prev_action
    FROM user_activity_log
)

SELECT
    client_id,
    DATE_TRUNC('month', hitdatetime)::DATE AS month,
    COUNT(CASE WHEN ("action" = 'login') AND (query1.prev_action = 'visit') THEN 1 END)
FROM user_activity_log
GROUP BY 1, 2;

ERROR:  missing FROM-clause entry for table "query1"
LINE 8:     count(case when ("action" = 'login') and (query1.prev_ac...